When granulating thermoplastic material, such as polyethylene or polypropylene, or pharmaceutical materials, granulating devices are often used in which extruded, melted plastic or pharmaceutical material is formed into strands in a die head.
Hot material can be pushed in a molten state through nozzles of a nozzle arrangement of a die head into a cooling fluid such as water or air that is contained in a cutting chamber. In the cutting chamber, there is a blade arrangement with blades that pass over the openings of the nozzle plate to cut the material strands so that pellets are formed.
On the die head side, it is necessary for the melt-conveying parts to be as hot as possible in order to prevent the melt from hardening before the strand formation by the nozzles.
After the strand formation, the melt material in the cutting chamber must be brought to a hardened state as quickly as possible by cooling. On the cutting chamber side, therefore, the housing parts of the cutting chamber must be as cold as possible in order to prevent the granules from adhering to them.
Therefore, there is often a large temperature difference between the parts of the die head and the cutting chamber housing.
At the points in which the parts of the die head and parts of the cutting chamber housing are in contact, the high temperature gradient due to the large temperature difference causes a correspondingly conductive high heat flow from the hot die head, through the contact point, and into the cutting chamber housing.
In order to counteract this undesirable heat flow, insulating materials can be used, which are inserted proximate the contact points to thermally insulate them from each other.
It is known in the art to separate a granulating head from a granulator housing by means of an insulation layer.
Prior art describes describes a granulating device in which a water chamber is connected to a nozzle plate, a seal can be inserted between the water chamber and the nozzle plate to insulate the heated nozzle plate in relation to the water chamber.
In order to minimize the heat flow, ideally insulating materials with the lowest possible heat penetration coefficient b should be used. The heat penetration coefficient b is given as the square root of the product of the thermal conductivity λ, the density ρ, and the specific thermal capacity c of the material:b=√{square root over (λρc)}
When selecting an insulating material, it is necessary to take into account technical requirements of the material such as material sealing, product compatibility, insulating properties, and the mechanical stability and load rating of the contact point.
These different requirements usually cannot all be concurrently optimized, so the selection of insulating material is often a compromise between these different requirements. As a result, in many instances, an insulating material which does not achieve the desired low heat penetration coefficient b is chosen.
This reduces insulating efficacy of the material, and allows for a significant heat input, which can be disruptive and undesirable.
One example of such a process is hot-cut pelletization with air as a process fluid, which is also referred to as air-cooled hot die-face pelletizing.
Based on the low thermal capacity of the process fluid (for example, air), it is not often possible to minimize the above-mentioned heat input to prevent the housing temperature of the cutting chamber from exceeding a beneficial value. There is thus a danger of granules not cooling to solidity in a desired time period, and adhering to the walls of the cutting chamber.
The heat input can also have a disadvantageous effect on the die head since heat is removed from the die head, thus causing it to cool locally at this contact point. This local temperature decrease can in turn result in undesirable temperature distribution in the die head.
This can also occur in underwater granulation, in which the process fluid (for example, water), due to its high thermal capacity, produces a relatively high heat dissipation. The water effectively carries off a heat flow traveling into the cutting chamber housing, which in turn results in more heat removed from the die head and intensifying local cooling of the die head at the contact point.
The object of the present invention, therefore, is to disclose a granulating device that overcomes the above-mentioned disadvantages.
Another object of the invention is to disclose a granulating device in which heat transfer between the cutting chamber housing and the die head is effectively prevented or reduced.
These and other objects of the present invention are attained by the present embodiments.
The present embodiments are detailed below with reference to the listed Figure.